As a method for manufacturing deionized water, a method of deionizing water by causing the unprocessed water to pass through ion-exchange resin particles (hereinafter referred to simply as “ion-exchange resin”) has been known. In this method, however, it is necessary to regenerate the ion-exchange resin using chemicals if the ion-exchange capacity is decreased. In order to eliminate such an operational disadvantage, an electric deionizing method which needs no regeneration whatsoever using chemicals has been established and commercially used.
This electric deionized water production apparatus has a basic structure of a deionizing chamber containing a layer of mixed ion-exchange resin as an ion-exchange material consisting of an anion-exchange resin and a cation-exchange resin, packed in a space between a cation-exchange membrane and an anion-exchange membrane. Water being processed passes through the layer of the mixed ion-exchange resin and, at the same time, a direct current is applied in the direction vertical to the flow of water being processed via both ion-exchange membranes to electrically remove ions in the water being processed flowing out of both ion-exchange membranes, thereby manufacturing deionized water.
JP-A-2003-334560 discloses an electric deionized water production apparatus having a deionizing chamber packed with a monolith-shaped organic porous ion-exchange material (hereinafter referred to from time to time as “monolith”). Unprocessed water is caused to pass through the deionizing chamber to remove ionic impurities therefrom, thereby producing deionized water. At the same time, a DC electric field is applied to the deionizing chamber to discharge ionic impurities adsorbed in the organic porous ion-exchange material outside of the system, wherein the DC electric field is applied so that the ions to be discharged may migrate in the direction reverse to the flow of water through the organic porous ion-exchange material. Because the electric deionized water production apparatus of JP-A-2003-334560 has a deionizing chamber with a large width and employs a monolith having a three-dimensional network structure as a packing material for deionizing chambers, the structure of the apparatus can be simplified and the costs for the materials, processing, and assembly can be reduced, as compared with the electric deionized water production apparatus in which a DC current is applied in the direction perpendicular to the direction of water being processed. In addition, since the monolith has a continuous structure throughout all the packed layers as compared with the ion-exchange resin particles, the monolith can easily adsorb and desorb ions, and accelerates transfer of adsorbed ionic impurities, thereby easily discharging the adsorbed ions. Thus, the monolith has an outstanding effect of being completely free from production of scale of calcium carbonate, magnesium hydroxide, and the like.    (Patent Document 1) JP-A-2003-334560
However, since only the monolith is filled in the deionizing chamber in the electric deionized water production apparatus of JP-A-2003-334560, the electric deionized water production apparatus has problems such as a small ion-exchange capacity and poor capability of accommodating itself to fluctuation in quality of the water being processed. In addition, the same as in the case of the ion-exchange resin particles, the monolith has a problem that the packing conditions change due to swelling and shrinking accompanying ion-exchange reactions when the chamber is packed with only a single type of ion-exchange material.
The mechanism of swelling and shrinking of the material packed in a container will be explained taking an ion-exchange resin as an example. The swelling rate of a cation-exchange resin is 7%, and the swelling rate of an anion-exchange resin is 23%, for example. The swelling rate refers to a rate of volume change when the form of an ion-exchange resin changes from a salt form to a regenerated form. For example, when a cell with a volume of 160 ml is packed with 160 ml of a regenerated-form (R—OH) anion-exchange resin and unprocessed water is caused to flow through the cell for a prescribed period of time, the form of the anion-exchange resin changes from the regenerated-form of R—OH to a salt-form of R—Cl, R—NO3, R—HCO3, and the like, and 160 ml of the volume of the anion-exchange resin decreases about 30% calculated as 160 ml/1.23=130.1 ml. As a result, some areas are produced in the deionizing chamber in which the resin is not filled, but only water flows. This causes a deflected flow of water, which results in an undue increase in voltage and ultimately makes it impossible to obtain a current flow required for removal of ions. On the contrary, when a cell with a volume of 160 ml is packed with 160 ml of a salt-form anion-exchange resin such as R—Cl, R—NO3, R—HCO3, and the like, and unprocessed water is caused to flow through the cell for a prescribed period of time, the anion-exchange resin changes into a regenerated-form of R—OH. As a result, a force to increase the volume of the anion-exchange resin from 160 ml to the volume calculated to be 160 ml×1.23=196.8 ml, is created. In this case, however, since there is a container for the deionizing chamber, problems such as breakage of the container due to concentration of force in the area in which the strength is the lowest in the deionizing chamber and an increase in the resistance to water flow occur. The monolith has the same characteristics to swell and shrink, and is subjected to approximately the same degree of volume change. In order to solve such problems of swelling and shrinking of an ion-exchange material, a method of previously determining the volume ratio of the salt form and regenerated form of the ion-exchange material to be filled in the deionizing chamber can be considered. However, it is impossible to determine the volume ratio before filling because the ratio of the salt form and regenerated form in the deionizing chamber varies according to the quality of the unprocessed water and the current efficiency in continuous operation of an electric deionized water production apparatus. Under such a situation, development of an electric deionized water production apparatus in which the problem of deflected flow and poor contact with the ion-exchange membrane caused by the swelling and shrinking of an ion-exchange reaction can be solved, while maintaining the advantageous features of the electric deionized water production apparatus using monolith described in JP-A-2003-334560, has been desired.
Therefore, an object of the present invention is to provide an electric deionized water production apparatus having a simple structure which can reduce material cost, process cost, and assembly cost, capable of accelerating migration of the adsorbed ionic impurities to facilitate discharge of the adsorbed ions and free from a deflected flow due to swelling or shrinkage accompanying an ion-exchanging reaction, and from poor contact with an ion-exchange membrane.